I'm not sure, but why not start by creating a "regular" boot.wim/install.wim, then convert them to something ESD? There are numerous softwares for this, and the WIM would be highly compressed during the conversion. Do you plan to customize the WIM by integrating updates, etc (WinToolkit is good for this)? Or is it a stock, unmodified WIM (MSDN, TechNet, retail, etc)?

I don't recommend to use boot.esd, use boot.wim, this is the usual way. I always see it this way even if there is an install.esd, I think you need to read and run first boot.wim to be able to read .esd files.

I have tested this making an install.esd from Win7 SP-1 install.wim and if you replace it in the ISO, you don't see install.esd, but if you replace this install.esd from 7 in a Win8.x ISO, then it installs without any issue. This is the proof .esd reading capability is in 8.x boot.wim.